This invention relates to an image frame composing circuit utilizing a color look-up table, in which a plurality of image frames are superposed in the order of priorities already assigned to these frames, to compose a reproduced image. More particularly, this invention relates to an image composing circuit of the type in which, for image frame superposition, "transparency" is assigned to color values, which are represented by three primary color signals, R, G, and B, and the picture elements (pixels) in the image frame with lower priority are displayed in the pixel display portion where the entry address data corresponding to the "transparency" in the frame with higher priority is stored.
Some recently developed image display terminals, such as Videoteks, employ image composing systems. In one such system, the image display terminal displays a plurality of image frames, which are superposed to reproduce the original image. In another system, a color look-up table is used to increase the range of displayed colors. In yet another system, the above two systems are combined, with the disadvantage in that the construction of the video memory output section is thus more complex.
A typical example of such a combined system is a CAPTAIN PLPS (Presentation Level Protocol Syntax) system. In the image display terminal based on this system (CAPTAIN receiving adaptor), three frames are used--a code frame for storing data such as characters transmitted in the coded form, a command frame for storing graphic data which is decomposed into graphic elements and transmitted in a series of geometric commands, and a photo frame for storing graphic data which is decomposed into pixels and transmitted in the form of pattern data. These frames are priority ordered for display, the code frame, the command frame, and the photo frame, in this order. The frame with low priority appears, as "transparency" only in the portion which is color-designated in the frame with high priority. Therefore, the portion where the photo frame is displayed corresponds to the portion where the code frame and the command frame are simultaneously color-designated "transparency".
The color designation method of this system is based on the color look-up table system. In the terminal of this system, the primary color signals are not directly stored in the memory of each frame; instead, an address (called an entry address) to enable the accessing of a color look-up table (CLUT) provided for each frame is stored in the memory of each frame. The entry address of each address has a 4-bit length, and can designate 16 (=2.sup.4) types of color designations. The CLUT is normally made up of a random-access memory (RAM). The intensities (Luminance) of three primary color components red (R), green (G), and blue (B) are set in each entry address. A feature of the CLUT is that when each primary color is set by 4 bits, a total of 4,096 colors (2.sup.4 .times.2.sup.4 .times.2.sup.4) can then be set. In other words, a great number of colors can be designated. Of these colors, 16 colors are selected for each CLUT. In the case of a plurality of frames, it is necessary to designate "transparency," as described above. Usually, when R, G, and B signals as set for the CLUT are all "0", "transparency" is designated.
In the terminal which is controlled by a microprocessor (MPU), the MPU, when the power supply is turned on, sets the default value for each color, which is preset for the CLUT, in the memory address for each entry address. The default value of the CLUT is so set that when the entry address is "1000", the R, G, and B signals are all "0", viz., "transparency" is designated.
In the image composing circuit of the prior terminal, a transparency designation signal of each frame necessary for frame selection, when the image is composed, is generally obtained from a primary color signal which itself is obtained by converting the synchronized entry address, derived from each address, by means of the CLUT. Since frame selection must be performed for each display clock, the access time of the CLUT and the delay time of the transparency-detect circuit and the frame select circuit are problematic. To cope with this, the terminal uses a plurality of latch circuits to absorb the delay time and synchronize these items of data. Since the primary color signal for each frame is 4 bits long, a total of 12 bits are required for the three primary color signals R, G, and B. This results in an increase in the number of gates in the latch circuit, thereby making the circuit construction more complicated.
All of entry addresses of each frame must be simultaneously input to the CLUT of each frame. Therefore, 12-bit address lines must be wired for each frame. This results in a large number of lines.
In view of this, there is now considerable demand for a image composing circuit using the CLUT, which is more compact and has reduced wiring, but which retains the variety of colorings at present in the CLUT.
Japanese Patent Kokai No. 60-205582, which is directed to that improvement, however, loses the feature of the variety of colorings, because a single memory for the CLUT is used commonly for the respective frames.